The embodiments disclosed herein pertain to refrigeration systems and refrigeration methods.
Ethylene plants require refrigeration to separate out desired products from the cracking heater effluent. Typically, separate propylene and ethylene refrigeration systems are used. Often, particularly in systems using low pressure demethanizers where lower temperatures are required, a separate methane refrigeration system is also employed. Thus three separate refrigeration systems are required, cascading from lowest temperature to highest. Three compressor and driver systems complete with suction drums, separate exchangers, piping, etc. are used.
Mixed refrigerant systems are known. In these systems, multiple refrigerants are utilized in a single refrigeration system to provide refrigeration covering a wider range of temperatures, enabling one mixed refrigeration system to replace multiple pure component cascade refrigeration systems. These systems are characterized by mixtures of components with sequential carbon numbers. These mixed refrigeration systems have found widespread use in base load liquid natural gas plants.
The application of a binary mixed refrigeration system to ethylene plant design is disclosed in U.S. Pat. No. 5,979,177 in which the refrigerant is a mixture of methane (carbon number 1) and either ethylene or ethane (carbon number 2). A binary refrigeration system usually operates with a fixed composition at given temperature and pressure conditions. However, such a binary refrigeration system is limited in the temperature range over which the system can be operated and therefore it must be cascaded against a separate propylene refrigeration system that provides the refrigeration in the temperature range of −40° C. and warmer. Therefore, two separate refrigeration systems are required.
U.S. Pat. No. 6,637,237 teaches the use of a single refrigeration system for all of the refrigeration requirements of an ethylene plant. The refrigerant used is a mixture of methane, ethylene and propylene (carbon numbers 1, 2 and 3). The system is capable of supplying refrigeration at temperature levels from below −140° C. to near ambient temperature level.
U.S. Pat. No. 6,705,113 teaches the use of a single refrigeration system, again with three components, utilizing a different process configuration than U.S. Pat. No. 6,637,237. However, both U.S. Pat. No. 6,637,237 and U.S. Pat. No. 6,705,113 require three components to supply the required refrigeration duties at the required temperature levels. Using three components means that the composition at any given system temperature and pressure does not correspond to a unique composition but rather to a range of compositions. Thus, the composition of the refrigerant mixture at any given point in the system can vary. While this variation is small and does not appreciably detract from the performance of the refrigeration system, it may be preferable to operate a system that has no refrigerant composition variation at a given temperature and pressure to more easily adjust to changes in ethylene plant operating conditions.